1. Field of the Invention
This invention relates to a process for producing a semiconductor substrate, and more particularly to a process for producing semiconductor substrates which are low-cost substrates having thin film crystal layers formed thereon and preferably suitable for use especially as substrates for solar cells.
2. Related Background Art
As drive energy sources for various instruments or as power sources systematically connected with commercial electric power, solar cells have already been widely studied and developed. In the fabrication of such solar cells, because of a demand for lower cost it is sought to form semiconductor devices on substrates which are as low as possible in cost.
Meanwhile, as semiconductors constituting the solar cells, silicon is commonly used. In particular, from the viewpoint of the efficiency of converting light energy into photovoltaic force (i.e., photoelectric conversion efficiency), single crystal silicon is the best. However, from the viewpoint of achieving large area and low cost, amorphous silicon is considered advantageous.
In recent years, for the purposes of cost reduction comparable to amorphous silicon and high photoelectric conversion efficiency comparable to single crystals, use of polycrystalline silicon has been studied.
However, in such single crystal silicon and polycrystalline silicon, processes conventionally proposed require slicing bulk crystals into plate-like substrates, and hence it has been difficult for them to have a thickness of 0.3 mm or smaller. Thus, the substrates obtained by slicing bulk crystals in this way consequently have a larger thickness than is necessary for absorbing light in a sufficient amount and can not be said to be completely effectively utilized as substrate materials. Namely, in order to make semiconductor devices such as solar cells lower in cost, the substrate must be made much thinner.
Recently, a method has been proposed in which a silicon sheet is formed by a spin process carried out by casting droplets of molten silicon into a mold. The substrates thus obtained, however, are about 0.1 mm to about 0.2 mm thick at the smallest and are still not sufficiently thin, compared with the thickness (20 .mu.m to 50 .mu.m) required for the absorption of light.
Under such circumstances, an attempt has been proposed in which thin film epitaxial layers grown on single crystal silicon substrates are separated (peeled) from the substrates, and the peeled films are used in solar cells so that a high photoelectric conversion efficiency and a low cost can be achieved (Milnes, A. G. and Feucht, D. L., "Peeled Film Technology Solar Cells", IEEE Photovoltaic Specialist Conference, p.338, 1975).
In this method, however, an intermediate SiGe layer must be put between the substrate of single crystal silicon and the growing epitaxial layer, followed by heteroepitaxial growth in that state, and further the grown layer must be peeled by selectively fusing this intermediate layer. In general, in the heteroepitaxial growth, a difference in lattice constant tends to cause defects or imperfections at growth boundaries. Also, in view of the use of different kinds of materials, this can not be said to be advantageous in process cost.
Thin, crystal solar cells are also obtained by a process disclosed in U.S. Pat. No. 4,816,420, i.e., a solar-cell fabrication process characterized by forming a sheet-like crystal by selective epitaxial growth, or lateral growth carried out on a crystal substrate through a mask material, and thereafter separating the resultant crystal from the substrate.
In this process, however, openings provided in the mask material are line-shaped. In order to separate the sheet-like crystal grown from the line seeds by selective epitaxial growth or by lateral growth, the cleavage of crystals is utilized to mechanically peel it. Hence, if the line seeds are larger than a certain size, they come in contact with the substrate in so large an area that the sheet-like crystal may be broken when it is peeled.
Especially when solar cells are made to have large area, however narrow width the line seeds have (about 1 .mu.m in practice), it is difficult in practice to obtain the desired semiconductor substrate if they have a line is length of several mm to several cm or a size larger than that.
Under such circumstances, it has been proposed to form a porous silicon layer on the surface of a silicon wafer by anodization, thereafter separate it from the wafer surface, fix the separated porous layer onto a metal substrate, form an epitaxial layer on the porous layer, and, using the epitaxial layer thus formed, produce a thin film crystal solar cell that exhibits good characteristics (see Japanese Patent Application Laid-Open No. 6-45622).
Japanese Patent Application Laid-open No. 8-213645 also discloses that a porous silicon layer is formed on a silicon wafer, a thin film silicon layer is grown on the porous layer, and thereafter the grown thin film silicon layer and the silicon wafer are separated from the porous silicon layer so as for the former to be used to form a solar cell. Also, residue of the porous silicon layer is removed from the silicon wafer from which the thin film silicon layer has been separated, and thereafter the resultant silicon wafer is reused so as to achieve a cost reduction.
In these processes, however, the thickness of the silicon wafer decreases with an increase in times of reuse. Thus the silicon wafer becomes difficult to handle, and there is a limit to the times of reuse. Hence, in this case too, it is difficult to say that the materials are effectively utilized.